As millions of Americans quit their jobs in favor of taking time off, finding a new job or even starting their own business, financial experts have a warning: Don’t lose track of valuable employer-sponsored retirement plans.

Neglecting such a plan, which can hold thousands of dollars after just a year of work, is more common than you might think. In the decade from 2004 to 2014, more than 25 million people who switched jobs left one or more employer-sponsored retirement accounts at a former workplace, according to a report from the Government Accountability Office.

Of course, these accounts can be recovered years after the fact. However, tracking them down later can take a lot of time and effort, and means you’ve probably missed out on earnings from neglecting to manage your investments properly.

In addition, if you don’t find any lost employer-sponsored plans by the time you retire, you could face penalties for failing to withdraw money regularly.

“We highly recommend people don’t leave money scattered all over,” said Gail Reid, a certified financial planner and private wealth advisor for Ameriprise Financial Services in Glendale, California.  

When to rollover

To ensure you don’t leave any money on the table, there are a few things experts recommend doing when switching jobs.

First, if you have an employer-sponsored retirement plan that needs to vest, you may want to wait to quit to ensure you get all the matching funds you can, said Reid.

When you start a new job, you have a few options concerning your old plan. You can keep the money there, transfer it to a new plan with your current employer or put it into an individual retirement account.

The process of moving funds is called a rollover and can be done through plan administrators to make sure you don’t get hit with added costs.

“If you do it the right way, there shouldn’t be penalties,” said Kaya Ladejobi, a CFP and founder of Earn Into Wealth in New York.

One reason to roll all your money into a new employer plan is that it’s helpful to keep everything in one place — it makes it easier to ensure that your investments are properly allocated, and update beneficiaries. Having everything in one plan can also come in handy if you’d like to take a loan against your 401(k), which people can do for things such as a home purchase, said Reid.

On the other hand, rolling funds into an IRA can make sense if you don’t have a new employer plan or would like to have more control over your investments.

Should you cash out?

It almost never makes sense to cash out a retirement plan if you’re younger than 59½, according to Reid. Having the money invested in the market will always yield a better return and avoid costly penalties before retirement.

In some cases, if you have less than a certain amount of money — usually $5,000 — you may be forced out of your former workplace plan. If this happens, you’ll have to roll over your account yourself, or your previous employer may send you a check.

More from Invest in You:
These companies have a 4-day workweek. What they’ve learned
The ‘Great Resignation’ is burning out those who stay
Use these resume strategies to stand out to recruiters

Of course, you don’t have to move accounts right away. If you have more than $5,000 in a plan, you generally can’t be forced out.

If you need time to sort through your next money move, that’s OK — as long as you don’t forget about an account.

“You can always leave your retirement account where it is; you just don’t want to lose track of the money,” said Ladejobi.

Tracking down a lost account

If you’ve lost track of any accounts, it makes sense to start getting them together and consolidated as soon as possible.

The best way to start is to make a list of your previous employers and see if you can find what company they use for their retirement plans, said Reid.

You can also search the National Registry of Unclaimed Retirement Benefits or Brightscope, which both have lists of lost or unclaimed accounts.

It especially makes sense to start consolidating your accounts if you’re over 60, said Reid. And, if you’re older than 72, you should have one account you’re drawing from to avoid complicating required minimum distributions — which sometimes must be taken from each individual account, depending on the type.

If you completely lose track of a 401(k) and fail to take the required minimum distribution, that can result in a 50% penalty, said Reid.

Even if you aren’t near retirement, having everything in one place can help you plan better for the future.

“You can’t know where you’re headed, if you don’t know what you have,” said Zaneilia Harris, a CFP and president of Harris & Harris Wealth Management Group in Upper Marlboro, Maryland.

SIGN UP: Money 101 is an 8-week learning course to financial freedom, delivered weekly to your inbox.

CHECK OUT: How to make money with creative side hustles, from people who earn thousands on sites like Etsy and Twitch via Grow with Acorns+CNBC.

Disclosure: NBCUniversal and Comcast Ventures are investors in Acorns.