Riddles, brain puzzles and mathematical problems

[PutraenusAlivius]

It's the mail guy. Because if he said he moved it between 2 and 3 then when they check it then it should be there.

13112221
Because you counted the amount of numbers in the previous numbers. First one is One "1", and so the second is 11, since it's 1 "1". The third is 21 because in the previous there are Two "1"s. Next is One "1", One "2". And so on.