There is a thief in my office

They started out by stealing fruit juice that was left in the fridge and grapes. I thought I was cleverer than them when I realised people in my office don’t like yogurt which I love. So I started to leave yogurts in the fridge. But then the thief decided to experiement and shortly the yogurts started to disappear.

Yesterday I bought plums and had one left over. I tied it in a plastic bag and left it in the fridge. This morning, the bag was still there and still tied, but there was a big hole in it and no plum.

 That was the final straw. I am now contemplating one of the following:

a. moving one of the cctv cameras we have in the office (members of the public use a part of the office lots) without anyone seeing and putting it to watch the fridge to catch the thief in action; or

b. going to the pharmacy, buying some laxatives, lacing something nice and stealable with them and planting it in the fridge. Then wait to see who is off work ill the next day!

 Any other options?