I would say it would be an unconstitutional seizure of your property. Constitutional search. However, they had a warrant for his property at that address. Regardless if there were different leases for the same rented space, they had the proper address.
Though for your property you claim they should have only been limited to "plain sight". Meaning if they saw a illegal substance in plain sight they could seize it. If they had probable cause that they saw your computer and it was a computer based crime they would have cause for a new warrant.
Granted, because a computer based crime can be done on any computer on the same connection or residence they should have had a warrant for all computers in the residence that could use that connection.
Where the fuzzy part gets is who all else can use a connection via wireless causing a search of innocent property.