Players appearing in team????

After the season update i got the usual list of contract renewals, and went about offering new contracts to all the players involved.
When i got to the last one, i couldn't renew as i apparently had a full squad???
When i looked, i noticed i had 23 players, yet i hadn't signed anyone in weeks (due to refusing to part with my own cash just to use one of the most basic of tasks in a game such as this).
I couldn't figure it out, until i noticed a 33 year old 3-star striker had suddenly appeared overnight.
I NEVER sign players over 22-23 years old as they are just not worth it, so i know for a fact i did not sign this player (in fact i didnt even bother to log in yesterday).
Also, as my players were still under contract yesterday, and this was telling me i had a full squad, it was IMPOSSIBLE to sign this mysterious donkey anyway!
It was easily sorted by sacking him, (couldn't sell as was retiring next season) and then i was able to offer the new contract to my last player, but is this a bug, someone hacking my account, or just another way for this game to somehow rip us off?? (not figured out a motive behind this one yet to be fair).
Just wondered if anyone else has had this before.
Also i notice it was a retiring donkey, at the time of contract renewals, and with a full squad.
I bet i never see a 5-star youngster appear in my last available slot at the beginning of a season!

You can sign players if you have a full squad, it just puts them in your squad page without contract, and you have 7 days to sign a contract with them.

You get all these old free agents showing up in your squad if your player retires or something.