I'm a bit confused by your question, so I will give my input based on how I've interpreted.
1 - Depending on the system you are using to file signature cards on closed accounts, you may not be able to easily purge them when your retention period is up. Clearly indicating the account has been closed and the date it closed on the card itself would help with this task.
2 - A signature card on a closed account that is not "voided" in some manner could present control issues. A dishonest employee could use the card to their advantage which may not be recognized easily if it were placed back into the "active" or "open" cards.
3 - Administratively, employees may be tempted to use an older signature card vs. a new one if a previous customer closes an account and comes in at a later date to open a new one (been there - seen it happen

). It could also result in a mess if cards are mis-filed and you do not have a distinction between closed and active cards. The integrity of the entire signature card system could be questioned if a closed card is found in the active cards - and vice versa.
These are just some issues that I could present if I were challenged with this question. I probably wouldn't consider this a "significant" finding and there may be mitigating controls to offset what I have just presented.
Always remember, the ICQ is a baseline for the audit. Just because they have answered "No" doesn't necessarily mean there are weaknesses. You just start from there, consider the risks and control objective, and then determine whether your bank has addressed them in any way.
I hope I have understood your question and provided some assitance. Good Luck!