Ricci was eager to stash something under Thurber's trailer. May have been drugs. May have been a gun. In any event, after Thurber caught him, he had to find another hiding place.
I'm sure there was some "dirty" urine test--or more than one--reported by the drug-testing people to his parole officer. But having a dirty urine is certainly not the same as being caught with the stuff. They don't arrest for dirty urine, unless a person is already on probation or parole. There has to have been some sign of drug use--or his parole officer would not have included that as one of the grounds for revoking his parole. BTW, to my knowledge, they don't have good tests for alcohol use, it's hard to detect unless they actually catch the person when he's just been drinking.
As for the drugs themselves, and the syringes or whatever, I can't believe he left them lying around. He wouldn't have wanted his great friend Mrs. Morse, or for that matter, Mr. Mitchell or Angela or anyone, knowing what he was doing. Or maybe he was using some of those "prescription medicines" he said he also liked. It's easy to get Lortab, for example--Utah Girl has discussed with me on these threads how dentists prescribe that for just anyone. It's a narcotic, I'm pretty sure, ask her. Then there was Soma in the house--not narcotic, but a nice high.