Dear list members, 

Prof. Walter Slaje very kindly pointed at a different meaning of saṃ+bhū. With his permission, I am sharing his message:

I think it is a passive-infinitive construction, with saṃbhāvita taking the position normally expressed by √śak, etc: ~ śakyate bhavitum, śakyaṃ bhavitum = "it is possible, it can be".
saṃ-√bhū expresses possibility (like √śak), so saṃbhāvita as a participle may form part of a passive construction as well. Thus, nirīkṣituṃ na sambhāvitaḥ would mean "he cannot be seen," literally, „he is impossible to be seen“.