They purposely appear to be everywhere in political campaigns — No-nonsense-looking men and women in dark suits and cool shades, coats open, hands always empty, wandering eyes inspecting the crowds every second but, strangely never resting on the candidate.

Some can be seen walking along rope-lines urging each spectator to keep hands out of pockets. Each one of them has a bright-colored lapel button, a different color from day-to-day. Automatic weapons you can’t see. And, of course, the curly cord running from one ear. More of them mingle inconspicuously within the crowd.

They are all Secret Service agents on protective detail.

It’s not cheap protecting candidates, however slight their chances of winning might be. And now comes a report that the Service’s major candidate coverage in 2016 should have been a good deal cheaper.

According to the Government Accountability Office, the Service overpaid four campaigns — Hillary Clinton, Donald Trump, Bernie Sanders and Ben Carson — an estimated $4 million in travel costs. Funny how we never hear much about government agencies underpaying for anything.

The GAO asserts the Secret Service used the wrong formula to calculate the amounts it should reimburse campaigns for its agents’ travel expenses and, perhaps worse, it knew of the mistake for months without correction.

The GAO reports the Service spent about $58 million on travel during the 2016 campaigns. Of that total, about $17 million reimbursed campaigns for agents’ seats on candidates’ charter flights.

The report did not break out over-payments to each campaign. But total payments included $615,567 to the Carson campaign for 107 flights, $2 million to Sanders for 159 flights, $7.1 million to Clinton for 1,317 flights and $7.3 million to Trump for 965 flights.

The previous payment formula, now amended, stipulated repaying the lowest commercially available first-class fare for that segment or a pro rata amount with the flight’s total cost divided by the passenger totals.

The Service said it is “working with all stakeholders in an effort to recover the overpaid amounts.” Last month it wrote all four campaigns seeking reimbursement with a deadline of last night. Checks are probably not in the mail — yet.

A Carson spokesman said he has no campaign at the moment, which isn’t the issue. But, hey, this is politics.

A Sanders spokesman professed puzzlement over the over-payments’ calculations and is seeking more information. Neither Clinton nor Trump campaign aides have yet acknowledged the letter’s receipt.