Any threat model tbh, your linux computer can be remotely used for botnet and you may not even find out (unlikely as linux is not targeted as much as windows simply because too few desktop linux users) Linux desktop’s only advantage currently is obscurity but that may not remain the case with rise of popularity in Steam Deck
No good solution but easiest thing to do is System monitor resource usage (CPU, GPU, Memory) check it when programs are closed and system is idle
Captcha is probably unrelated to botnet, it can be from your browser (most privacy focused browsers like LibreWolf, Brave, Hardened Firefox get this captcha problem) or your VPN/proxy if you use one