Hello and thanks for trusting me to help you and Sid today. I'm a veterinarian with over 25 years experience and would be happy to work with you.
The scenario that you describe can be worrisome, especially after he has just been through his weekend ordeal. The chance for re-obstruction is very real, but the emergency vet is correct that it may just be irritation of the urinary tract that he is experiencing now and not re-obstruction.
Buprenorphine can cause various types of reactions in cats. Some will be become lethargic, while others will become almost dyphoric (very spacey, disoriented, slow moving). Unfortunately, even if this is a response to the medications, if he is hiding, he is not eating/drinking well and not using the litter box, so it will be hard for you to monitor if he is passing urine or not.
I would keep a very close eye on him over the next few hours. If his behavior is due to the meds, the effect should diminish over time and hopefully he will come out soon. Try to encourage water intake so that he can produce urine. As a worst case scenario, if you are uncertain about the state of his bladder, a quick trip to the vet will be all that is needed to see if he is obstructing again or if he can produce a good stream of urine.
I hope this is helpful. My goal is to provide you with excellent service – if you feel you have received anything less, please reply back as I am happy to address follow-up questions. Kindly rate me when you are done. Thank you for allowing me to assist you.