Possible, but seems a little unlikely. If it's a 3-digit code, that's 1000 possible combinations (000 to 999), and if it's a 4-digit code that's 10,000 possible combos -- frankly, I don't think Melon's got enough dexterity to enter that many codes quickly enough to get into the apartment in a reasonable amount of time. (Like, less than half an hour.)
Besides, Roko said Melon knows her door code, and so presumably she knows her own code too.
I used to work as an auditor in a hotel. The first thing I had to do every night was manually copy the previous year's data onto the current worksheet (until I got tired of having to do that every day, and I wrote the code to
automatically pull the data from the previous year's
file). It was roughly 350 fields with numbers and decimals, and I could enter them all in less than 5 minutes (yes, that really
is faster than one per second). We know that Roko's door code is
only three numbers, so assuming that Melon's fingers are at least as fast as mine, she could easily punch in all the possible combinations in less than 15 minutes.
The problem is, all the electronic locks I've ever used will lock you out for a prescribed amount of time (usually between 5 and 15 minutes) if you enter more than a prescribed number of wrong codes (e.g. five tries). I even had one that would lock you out
permanently if you entered ten wrong codes before entering the correct code (it locked you out for 15 minutes after the first five, and then you got five more tries before it locked you out permanently). Then the only way to open the lock was to enter the master code, which was a ten-digit code. So regardless of how fast she could enter the codes, it's still almost impossible to accidentally open someone else's door (unless of course she knows the master code).
That leaves the only other possibilities being that A) Melon can electronically "crack" the code without actually entering it (hope Roko doesn't find out), or B) Elliott didn't lock his door after he came in - which is far more likely.