pen69 is inviting you to join giffgaff
If you order your free giffgaff SIM card through this page, you'll get £5 extra credit for free when you activate your SIM.
Note: You will need to top-up by a minimum of £10 to activate your SIM card.
The above clarifies that philip is correct in saying the sim (if topped up by only £5) was not connected to anyones account.
But, if you are certain, do contact an agent and get them to double check.