This comic claims that it was to prevent people panicking and attacking the crew. However, Vostok pilots were supposed to land separately from the capsule. Are there any photos supporting this claim? Which ships used such label?
Yes, the Soyuz TM-10 capsule had such a label next to the exterior hatch, according to The Bloc on the Block by Jeffrey Kluger in the April 1994 issue of Discover Magazine:
Then there were lots 154 and 175, the Kosmos 1443 and Soyuz TM-10 space capsules. Sotheby's hoped to get top dollar for these artifacts, and the catalog included photographs of some of the ships' more interesting details in an effort to spark the bidding. (One photo showed the message stenciled in Russian and English next to the Soyuz's exterior hatch, in case the spacecraft was found by a passerby before recovery teams could reach it: MAN INSIDE! HELP! OPEN THE HATCH! TAKE THE KEY! PUT INTO THE HOLE!)
There is a picture here, for those interested. (I won't be including the picture in the post due to copyright restrictions.)