Pretty sketchy when you can't trust people in your office with a darn betta. Of course, who knows it could be someone on the maintenance crew, some employee's kid they had to bring with them one day, etc. I would mention it to the higher-ups, because if a fish is getting stolen, chances are other things are, too (even if it is just people's mugs, office supplies, etc. - stealing is stealing). Besides a betta, personally a dwarf puffer would be my other choice for a 5G office tank. But as nguyen said, if you are really worried about another fish getting taken, maybe just a few guppies or something really cheap.