A federal agent pretending to be a trad? It would take years of training to learn not only the doctrines that we believe, but to memorize the prayers and learn all the other things that we pick up over the course of a lifetime, that would be necessary to make a convincing act of being a trad.
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Do you really think spooks take classes at Langley on which saint to pray to to help your cat get better, or where the Council of Trent was held, or the story of the 40 Holy Martyrs? Or how much of an indulgence you get for saying the rosary? Any trad's head is crammed with those and similar facts, which as I said could only take a lifetime to acquire.
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The idea seems borderline impossible, so unless the person you suspect of this is wearing a discreet earpiece and seems to be receiving verbal cues from it when he's talking to you, I wouldn't worry about it.

It wouldn't take years of training. All you have to do is sit in the pew and just keep coming until people get used to you. However, I don't see why they'd bother. 

USA is like East Germany. There is an American Stasi with agents, auxiliaries and snitches in every organization, chapel, forum and Walmart with sweet and innocent-looking old ladies working the registers who report what targeted people are saying after probing them with "small talk". They match the talking shopper with the credit card used, and there begins a new file and surveillance on an American citizen. Many neighborhoods have snitches, too. It's a lot bigger than you realize.

Heard a trad Catholic story in a Cincinnati chapel, years back, where infiltrators got in with the chapel pro-lifers.

After some time, they used their influence to try to provoke the group into anti-abortion violence.

In the same chapel, one of the trad ladies, a Jєωess convert, appeared such a deeply knowledgeable trad Catholic, she could have led her own chapel ladies club.  Later, she went off the deep-end and turned on Catholicism.

The point I'm making, these infiltration tactics are all very possible and even more so now.

But because of false brethren secretly brought in, who slipped in to spy out our freedom which we have in Christ Jesus, that they might bring us into bondage ~ Galatians 2:4

I think in most SSPX and other Trad chapels the Feds could find a stool pigeon pretty easily.  That is my impression after going to TLMs for the last 47 years. 

This is probably nothing to worry about but is easily dealt with. Don't discuss certain topics with anyone you haven't known for over a year. If a new parishioner keeps trying to turn the conversation to ....controversial matters that could attract the wrong kind of  attention  then avoid this person and calmly let other people know that they should watch what they say around him. He is probably a nut but to be on the safe side, silence is golden.

 Haven't known for over a year?  If I haven't known you since I was like 5, you're doing well if you get more than a good morning out of me.  Some people are more social I guess.

Not federal agents, but people were speaking about spies from the diocese at the indult reporting anything the priest says that was too traditional. There may be spies at the SSPX too. 

How do you know he does or doesn't?

or the guy who has his phone in his pocket like 95% of the entire human race. some of you think we live in stalinist russia

None of my business, but why would the feds be interested in your confessions?

Think about dragnet surveillance for one moment and why confessions would be exempt from big tech's collecting EVERYTHING