Doubts

If I am doubtful (due to lack of concentration/absent mindedness) as to whether I walked in a carpeted room with napaak slippers and there is no visible sign or smell of napaaki on the carpet, do I regard the carpet as paak to walk on with wet feet and to perform Salaah on, or does it have to be washed? NB: This particular type of doubt has occurred at least twice previously. It is habit to remove the slippers prior to going in to the said room, because I perform Salaah in that room, but as I said due to absent mindedness I could have walked in that room with the slippers, and although there is no visible sign or smell of napaaki, if it was present in the form of a few drops of urine, it would be impossible to tell.