Just a few random thoughts...
1. When was her trip scheduled for? Depending on the dates, I'm not sure her bf would have even been able to attend. Didn't he have a trip slated to go to another country?
2. As for a bf normally parking in the garage...I see nothing odd about that. When I used to spend nights or even just visit for the evening at my ex-bf's I would park in his garage. No real reason, I just did.
3. Him not staying with her on a regular basis. I can see that. When I was dating my husband, we maintained separate places. Some nights we would stay over at each other's but mostly not. Until he moved in with me, then obviously we spent every night together. I do find it very odd that she 'sent him home to study' though, unless there were a good reason for that. I keep thinking maybe she was expecting other company, or even just wanted to get her things in order for her trip. Still, odd wording there.
4. I would not even entertain the idea of calling the police prior to going in to check on a loved one, friend, etc. unless I had a dang good idea they were dead inside. Nope. I have a hard time thinking of any circumstance I would do that. Even if I suspected a friend had committed suicide, I would never think to call police first. Because what an embarrassment it would be to them if that was NOT the case. If I am understanding it correctly, that he did call before he even entered, that sends a huge red flag to me. Unless...he saw blood spots, etc. somewhere before going in. Just a lack of her answering her phone for a day would not be enough to make me think this was a major event.
5. I can, however, see him telling the police he hadn't touched anything. If he did, indeed, phone police before entering they may have told him not to touch anything and he was just relating that back after he found her that he didn't touch anything. Even if not, I can picture myself saying that straight away. I have also seen stories where the killer is the one that does 'find' the victim and right off the bat starts explaining that they DID touch stuff, kind of a pre-emptive reason why their prints, DNA, etc. might be there.
6. Dangit, I wish we could hear that 911 call.