a contemporary document about the second Jesuit 'mission' / terrorist infiltration (depending on how you look at it) is Philip Caraman - John Gerard Autobiography of an Elizabethan.
Good read - spy's manual and deals with the 1600-1610 years and was written around about 1605. The now St Nicholas Owen is a saint but then was code-named Little John and was not only John G's man servant but also priesthole builder extraordinaire. Examples of his work (including a hole within a hole) exist at Harvington Kidderminster.
Now the thing about the secret signs you mention is that in 1535 basically everyone was RC and the extreme protestantism of Edward VI was reluctantly accepted. so even in 1570s and 1580s everyone would know the old symbols. So showing Romish symbols to a stranger who was a prod would just get you into prison.
a bit like walking up Shankill Road shouting Holy Mairead mudder of God !
If you goggle secret catholic symbols - they all look like rather obvious christian symbols.
The book above is worth a read (but perhaps not at £45)
The house where Gerard is, gets raided by the pursuivants (= secret police) and so they take refuge in a room, and all hang onto the latch, which is tried by the Police. The lady of the house says - Oh! I must have locked it ! and they dont notice there is no keyhole. She goes off to get the key and they follow HER suspecting funny stuff. Later they forget they havent searched the room......