A pair of Dock bug (Coreus marginatus)  on Great burnet (Sanguisorba officinalis).
The bug can be frequently found on the plant, which is not a host or food plant. The nymphs use primarily sorrel (Rumex spp.) . Adults feed on knotgrass (Polygonum), sorrel (Rumex), klit (Articum), plumeless thistles (Carduus), plume thistles (Cirsium), cottonthistle (Onopordum) and sow thistles (Sonchus) .
The relatively large number of individuals and pairs found on the plant makes me think that it helps the bug in the process of locating a partner. The bugs use vibrations to communicate. A female will produce them and the male will listen and follow them to the source. They use plants to amplify the vibrations and by doing so extend the range of their communication, see also chapter 3 in the species post of the Green shieldbug.
References1 Nederlands Soortenregister
2 Pekár, S., & Hrušková, M. (2006). How granivorous Coreus marginatus (Heteroptera: Coreidae) recognises its food. acta ethologica, 9(1), 26-30.