My bet would be baking soda or vinegar, or a disínfectant.
I imagine the carpet was used because you went away, and the cats have not fully got over the death of their brother. My old cat refused to leave our garden after his brother died unless I went with him, even on to the banking that they had more or less regarded as theirs, although it is not part of our property. He also stopped mousing for nine whole weeks and I faced the prospect that we might loose him too, but then he took up hunting again. Nine weeks was apparently the period of mourning he found suitable. He survived his brother by seven happy years.
An earlier cat - a rescue - was so distressed when DH and I went away for a weekend three months after taking her and her brother in, that even although our son, whom she adored, was staying in our flat while we were away, she pulled my bathtowel off the rack and wet it until we came home again, when she reverted to using the litter tray. And she spent the first night we were back home, snuggled down between DH and me in bed.
This behaviour never occurred again, even when we went on longer holidays, so your cats' accidents may too be an expression of insecurity and not re-occur.
Good Morning 1st May 2026 "May Day"
Tuned To 'The Archers' For The First Time In Months.
Backseat Driver, Former PM Tony Blair Reckons The Triple-Lock...

.