I think he means stealing the tank, as in she just does it without thinking rather than deliberately.
Although it doesn't really work without the か at the end, there's also the possibility of him going "You are doing this on purpose, aren't you.", which either implies that he thinks she cried and guilt-tripped him on purpose, or that he thinks she's returning the tank right in front of him now just to make him feel guiltier.
Although it doesn't really work without the か at the end, there's also the possibility of him going "You are doing this on purpose, aren't you.", which either implies that he thinks she cried and guilt-tripped him on purpose, or that he thinks she's returning the tank right in front of him now just to make him feel guiltier.