"Assuming users can be identified, Russian forces can lawfully attack them and would not need to consider their presence if targeting nearby military objectives". Use of the app is "direct participation in hostilities". Use of the app is "lawful, but...". https://lieber.westpoint.edu/civilians-reporting-cell-phones-direct-participation-hostilities/ Ppreviously, I analysed and highlighted the technical-legal issues due to armed conflict and the use of modern technologies, like apps, during war time. Civilians may loose their protections... https://www.wired.com/story/smartphones-ukraine-civilian-combatant/
It seems that the definition of "direct participation" was already very much subject to precedent and complicated. https://www.icrc.org/en/doc/resources/documents/faq/direct-participation-ihl-faq-020609.htm provides some interesting nonobvious examples
Interestingly, if one reads the interpretation provided in https://www.icrc.org/en/doc/assets/files/other/icrc-002-0990.pdf (specifically pp. 52-54), it seems to imply that setting up automatic observation posts that will report enemy movement should be considered to be _in_direct participation.
