The most famous medieval heist is [Richard of Pudlicott’s] (https://en.wikipedia.org/wiki/Richard_of_Pudlicott) burglary of Edward I’s royal treasury at Westminster in 1303. Here’s a link to a 100-year-old [lecture about it by T. F. Tout] (https://archive.org/details/mediaevalburglar00toutuoft), who was a respected historian. He has a list of sources at the end that may lead to other heists.

In edition to the example provided by /u/Whoosier, another classic medieval 'robbery' can be found in Einhard's Translatio of the relics of St. Marcellinus and St. Peter. The text of course has to be read carefully. According to Einhard the saints want their relics to be moved! And of course they aid the thieves, who whisk them away to be consecrated in Einhard's new church where the saints will feel right at home.

For a more in depth treatment of relic thefts and other saint related shenanigans, Furta Sacra: Thefts of Relics in the Central Middle Ages by Patrick Geary is the go-to work on the subject.