What do we do if SETI is successful?

'In fact, the author has heard from serious U.S. SETI researchers that they are convinced that “men in black suits” will appear at their laboratory door the moment a detection is confirmed.'

Source: https://arxiv.org/pdf/1701.08422

This basically is just demonstrating how people very very good in their field can still fail Civics 101. Men In Black were some funny movies back in the day, but they were just movies.

"The term is generic, as it is used for any unusual, threatening or strangely behaved individual whose appearance on the scene can be linked in some fashion with a UFO sighting."

Some stories dont even posit them as being from the government, just designed to give that impression. Some reckon alien hybrids. Even in the MiB films they were separate and just controlled the government largely.

https://en.wikipedia.org/wiki/Men_in_black

Its a general fear of government intervention.

If you are asking on what legal basis this fear is founded. I don't think there needs to be one. Lots of governments do illegal stuff. Suits are the classic G man look. They dont need "Jurisdiction" to dress in a suit and harass SETI.

So, that most SETI of all SETI people, Carl Sagan posed the role of the security apparatus in Contact, in the form of NSC head Michael Kitz. The film version gace superficial treatment - Kitz locks down message data, and at the end repudiates Arroway’s visit. In his book version, however, the state apparatus is much more insidious. The astronauts (plural, multiple countries rep’d) are threatened with having their psychological reputations destroyed if they ever utter a word that their encounter ever actually took place, or contravene the governments’ (plural) line that the intended journey had failed. Arroway takes extraordinary measures to make sure her hidden testimony will get out ‘should anything happen to her’. Sagan had a security clearance. One is left with the impression that maybe he wasn’t just making up conflict for dramatic reasons.