Just look at your cell phone bill every month, all the calls are on there, it'll show up plain as day, there's no way to hide that it's logged traffic from the provider. Generally speaking the only people that should worry are those that have something to hide.
Keep in mind in the US at least doing something like that to someone elses phone without their consent could be breaking state and federal laws and could be legal grounds for felony prosecution. For examples if the spouse was stupid enough to confront you with it directly rather than feed the information to a private investigator that would be able to provide legal sources of the information any information they obtains from the use of a cell bug like that could be used against them for felony wiretapping violations, not 100% sure on the legalities.
Even if the phone is legally in their name if you're using it and they know that it violates wiretapping laws I think. Never looked too far into it though. Don't care to, not really worried about it myself. If you find yourself suddenly in a situation which for some obscure reason requires absolute 100% garunteed protection from a cell phone, wrap it several times in a sheet of tin foil making suring you curl the edges to be overlapping and press them firmly. 100% cell phone signal proof.