Help the forgetful professor retrace his steps by picking the most logical phrase.

I suppose I _____ my glasses in the staff room, but let me check my coat pockets just in case.

The correct answer is might have left.

We use might have (or may have / could have) + past participle to express that something was possible in the past, but we aren't completely sure. The phrase "just in case" shows that the professor is still uncertain.

If he used must have left, he would be absolutely certain and wouldn't need to check his pockets!