leeandjoanne you cannot get fired for this so relax firstly. He/she may just want to have a chat but you need to be prepared to make your case. Nobody was there to relieve you , that is their problem and not yours'. Perhaps you can say that if it is a problem in future, can you be reassured that you will get paid for the 15 mins and you would be then happy to stay.
What if you had to be out of the door on time; they need to be letting you go. If he is having a go at you logging off I would just plead ignorance that you simply didn't know.
I have been in positions in my life where little bossy managers have scared the daylights out of me too and it turns out they don't have the right to do an awful lot unless you commit a huge error and this was not a huge error in my book. Let us know how it went - maybe he wants to promote you....look on the bright side it could be unrelated.