send out an office-wide email stating that you have had reports of recent theft from the community fridge... include in the email a made-up-story about how the pregnant female has been placing different concoctions of hormonal/vitamin supplements into her drinks, and that the culprit should fess up so that you can offer medical assistance before he starts producing milk from his newly developing tits and his hair starts falling out...
This will either:
1. get a full confession
2. stop the theft, at least until the lady has her baby
This is 10,000 kinds of awesome!