I suggest you check again, I can't imagine where they went. I don't believe there is a big problem in leaving them in a couple weeks beyond the recomended time. But the real problem is that too many have been leaving them in over the winter and the mites are becoming immune to that medication. Another problem is that you definitely do not want to have these strips in the hive when the bees are filling up supers. It will contaminate the honey. This is strong stuff and you absolutely do not want to be consuming or selling honey tainted with this. I suggest you search again and if they are not there I'm stumped as to where did they go?

Check the bottom frames of the top box. A while ago I had the bees glue the top of the strip to the bottom of the frames and when I lifted the top box off the strip came with it. Like you I looked all over for that strip. Finally found it all crunched up under the top box.

No matter what, you're in violation of the regulations. The hive needs to be dismantled until you can find the strips.