Somebody’s been dipping into your 401(k) plan without your knowledge.

Hidden management commissions, obscure enough that few employees can even find them, are a growing problem inside the universe of company-sponsored retirement plans, according to a recent story in the Los Angeles Times.

It’s a given that mutual funds charge fees for management and other expenses. Since most 401(k) plans rely primarily on funds to grow employees’ savings, those annual costs are typically disclosed and should be easy to figure out by looking at your account statement or a fund’s prospectus.

But a more insidious form of surcharge also is being levied and employers don’t mind because it’s the worker that pays these extra fees.

Plan providers can be mutual fund companies or private specialists, and some charge each contributor a commission on his or her retirement account in addition to the cost of investing in a particular mutual fund offered through the plan.

But rather than deduct that annual fee in cash, they often take their cut in mutual fund shares, sometimes in quarterly installments, making the transaction almost impossible to locate since most plans invest contributions and reinvest dividends as they come in.

To find out what’s missing you would have to keep an eye on the fractions of shares you should have in your account.

And these extra fees can add up because the larger your 401(k) nest egg, the larger the annual commission. The more you save the more they take.

To make matters worse, there isn’t much you can do about it except to badger your employer to get you a better deal.