St. Anton is a patron for finding lost things. It means he is a good candidate to be a patron of geocachers...
The church is very interesting and very beautiful. It is unique, because it is one of the very few churches in Warsaw on the left side of Vistula shore, which was not damaged during II WW. Most of the others had been totally destroyed and rebuilt (like those in Old Town).
St. Anton's church was built at end of XVII century, as a part of monastery founded by Stanislaw Herakliusz Lubomirski. It was important place on religious map of Warsaw in XVIII and XIX century.
Interior of the church is a really beautiful example of baroque style, unfortunately it is closed most of the time, and can be viewed only through a firm grating in the entrance. It is a good idea to make a short walk in church garden (former cementery) too.
Cache is hidden outside the church wall, so it may be taken at any time of day or night.
EDIT: previous cache has been lost, so it has been reactivated. Size, coords and hint - updated.