For several days I noticed my ec decrease. For example I have 6700k ec before I quit the game. The day after I have 6600k ec. So I sale some stuff. I quit with 6700k ec. The day after. Again. Ec decrease. I have 6600k. What's wrong with my bank? How is it possible?
Your Account might be compromised.
There are several known situations like yours.
They milk your account steady and slowly, hoping you don't notice.
If you then happen to get a great drop worth millions, they take everything ... sounds ferengi.
And some doff assignments give small EC rewards. Your explanation and this can mean the account tends to fluctuate.
Now if the OP is saying he doesn't do doffs, doesn't trade, doesn't do anything but admire the 6600k in his bank and it keeps moving up and down.. that's strange.
Can shapeshifters pose as credit cards in STO?
Yes but considering where I hide mine I don't imagine they would want to.
Honestly without more information theres no way to be certain. I would just change my password on my game and email and see if it stops. Also file a support ticket it may be a data error.
As a time traveller, Am I supposed to pack underwear or underwhen?
Not everything you see on the internet is true - Abraham Lincoln
Doff assignment costs are subtracted at the beginning of the assignment, not during or at the end of it.
It's a known issue that accounts are accessible. Change your password now.
Some will say to file a support ticket but it's not worth it. They can only go back a few days. If they reinstall your account from a backup from a few days ago, that's only a little bit of money that will get returned if they've been taking the money for months and you'll lose whatever you've done for the last few days of game play.