My father is spying on my mother

Although I'm grown-up I still live in the same house with my parents and sister. About 2 and a half years ago my father had a severe heart attack. Since then he has become negative towards my mother (more) and myself and sister (less). He has closed a lot to himself and does not discuss anything, in fact in the beginning he would't even speak to us anything more than "hello" "goodbye" etc. About 2 years ago my father came to me and revealed that he had caught my mother cheating on him. This was in fact not true, as from what he said it appered that he only had this idea but he did not actually have any proof. My mother refused that this indeed happened and gave to him an explanation, however this did not change this idea he had. I should say here that I have a close relationship with my mother (and my sister) we are more friends than family and we discussed about this issue. We all noticed that my father had developed a sort of paranoid/obsessive behavior and depression after the heart attack and tried to get him to visit a therapist. He however refused to do so as he did not believe he had any problem. His relationship with my mother became estranged, however he did not move out of the house, as I would have expected, if he wanted to get a divorce. For the last 2 years they speak only the basic, however the "cheating" issue did not came up again. They have become let's say housemates. About a week ago, my mother discussed with him about a friend/acquaintance (woman) of his, for whom she had heard that she was a crook and was stealing money from a lot of people. She tried to warn him about this in order for him not to be cheated by that person. Anyway, since that day I accidentally found out that my father had purchased spying equipment (2 cameras and 1 gprs vehicle tracker) online. He does not know that I know. I have not told my mother yet, however I really don't know what to do. From one hand this situation has been going on for 2 years, so it does not make sense that he wants this equipment for my mother. On the other hand what would he want this if it's not for my mother. I really don't know how to react.

Thank you for your reply. We have spoken with his doctor from the beginning and explained the situation, and the doctor agreed with us that this is most likely depression. The thing is that he cannot prescribe him antidepressants without him at least insinuating a problem. When my father visited the doctor he stated that everything is great etc. etc.