I'd draw a tic-tac-toe board, place an X in the middle square and write "Your turn"
Depending on how long I was stuck in there, my next message would be "That's no Safe-House, it's a Space Station", then "The Safe House is a lie" and finally "Argh! So bored!"
Assuming I got rescued, my last message would be the date I entered the Safe-house, the date I got rescued and the following message "Caught a level 5 Rattata shortly after entering safe house, it is now a level 99 Ratticate. So, yes, you will be here a while. I left you some crossword puzzles in the First Aid Kit."