、。
I think there was probably a serious flaw in the lookout.
Literal
Probably, surveillance [in-に] serious flaw [subject-が] existed [it's not the case that-んではないか][quotative-と] am-thinking.
Formal investigative/analytical register. おそらく 'probably, in all likelihood' is more formal than たぶん. 見張り 'lookout, watch, surveillance' is a noun from the verb 見張る 'to watch, keep watch over.' ~んではないか 'isn't it the case that? / perhaps X' is an indirect suspicion marker — softer than assertively claiming X. 考えています 'am thinking' in the progressive form marks the ongoing state of holding an opinion, rather than a momentary thought.