I know its a dump question but really some times Markus go out for days or get sick and there is no one know about her. even after Erin did know about her she couldn't bring her food because Markus don't know that she knew.
sorry about my English
I felt that we are supposed to think "ONLY Markus"... but you're right, that seems rather problematic if he is away for more than a day or two (as he is at several points).
Thinking outside the box, I would say the only logical answer is that he has given Megan some "emergency" rations or something to last a while if he is forced to be away from her.
That still leaves for me the question... what if Markus died? Say, killed by the sniper in 'Tripwire'? As nobody else knows about Megan, she would simply starve to death? Not a nice way to go for someone you love. I know he was worried about telling anyone, but surely this crossed his mind at least once? Or maybe he knew deep down that Megan would probably kill herself if it wasn't for him being around anyway (just my speculation, that last part)?