I was attending Mass in an independent location. The priest was a well known defender of pre-Vatican II Catholicism, and the location of Mass was somewhat widely know. I had known this priest on a personal level for some time, so I was familiar with his community.
One Sunday, I was just outside the door taking care of an infant during communion. (side note, sometime I notice what I refer to as a "tunnel sight" - it is like in a crowd, sometime people are positioned in such a way that I gain an unusually long line-of-site through the crowd to some person or object (like seeing a single distant tree through a forest of trees) - at any rate, so I see through the crown to a person receiving communion, but as this person rises and turns from the communion rail, I see the host drop into the open missal and then the missal is closed. This person was joined by two others, a male in front and a female behind as they were walking for the door. I had never seen any of the three before. I stepped forward as they approached the door, but the male in front had some sort of effect on me, like being frozen in place. I remember looking in his eyes as he passed and feeling chilled. I was not comfortable with the situation.
My thought is that as true Mass locations become fewer, agents of evil that prey on the elements of our Faith will be encountered on a more frequent basis.
If the devil sends his agents to a traditional mass, it's proof of Christ's Real Presence in the Blessed Sacrament. Satan isn't stupid. He won't stand for a counterfeit sacrilege.