If an off-duty officer at a party accidently finds something illegal and makes an immediate arrest, would the evidence be excluded from the court?
Lets say that the evidence was six marijuana plants. The homeowner asks the off duty officer to get some ice from the basement; he cannot find the icemaker and opens a door and finds the marijuana plants.
The search was legal and there was no illegal or improper acts by the officer; therefore the marijuana plants discovered in the search would be admissible in court. The officer's status as off duty is of no consequence.
The issue turns on the fact that the officer was asked to retrieve ice from an ice maker and told where he would find it. The officer was acting in good faith and his discovery of the marijuana was apparently completely accidental. He had the consent of the occupant to enter that area of the premises, and thus was acting lawfully. Had the officer been snooping, or looked in an area where the ice maker could not be--say a tool chest--then the situation would be different. Under the present circumstances, however, the discovery of the marijuana plants was made while the officer was acting in good faith. Thus there are no grounds for exclusion.
Although there is case law of whether an off duty officer working at a second job, say as a security officer, acts under color of law or not, such is not legally relevant to your facts.
We could refer to the officer as a "social guest". Since a social guest is not trespassing, even if under color of law, arguendo, he has a right to be there.
The officer has the EXPRESS permission to get a party item. Even if the POT is discovered "inadvertently", that is, by accident, it can be seized as contraband. While there can be a slight question of whether a warrant should have been obtained to seize it, this is probably without merit, as you suggest.
Review the recent Kentucky v. King decision from the SC about the seizure of contraband due to the destruction of evidence theory. Sure, King dealt with a warrantless entry, and the officer here is not acting under color of law in the so called search, nor is he entering a home without a warrant, the same applies.