How can I tell if my two friends are hooking up secretly?

This all started on how my guy friend acts SUPER close with my friend name Lisa. He will blow off anyone just to be with her and even lie on his whereabouts to come find out he was hanging out with her. Lisa has a boyfriend, some things happen to him lately where all he has is Lisa and that he wants to kill himself. Which number one reason why this all concerns me. Suspicious things been happening between lisa and my guy friend david like when we went to the movies they were all hugged up on eachother and whispering to eachother and wouldnt let anyone know what they were saying even when were in the car. But anyway as the movie was over and were walking out they pretended to follow behind me and my other friends that we were with, when we went out the theater sitting there for like damn near 10 mins saying “where are they”. When I went back in it looked like they were kissing and when I said something they seemed shocked an immediately moved there heads away from eachother wanted to leave. Then another time when we were all hanging out including Lisa’s boyfriend, it seem like they waited for josh (lisa’s boyfriend) to go for abit so they could be alone. Josh went to the restroom and soon after that david got up and left me in the room alone while i was scared because of something recently happend so I didnt wanna be alone especially with a baby that is not mine. He said that she was giving him stuffing from a turkey on his plate and heat it up for him so he kept her “company” in the kitchen, wierd thing about it is that they kept whispering and where seem to be hiding on the other side of the kitchen so if you stand and look at the kitchen entrance you cant see them, and they kept whispering then it got quiet like they could have been doing something and not ONCE did she go over to the microwave where she was to post to be doing which is near the entrance and i would have clearly saw her do. When josh got out the restroom and popped in they quickly went back to talking in a normal voice and moved away from eachother and then david came back to the living room and had no stuffing on his plate. So what were they doing then???? I’m scared for josh because this will devastate him or even make him kill himself if its true. Also we were all apost to hang out last night but mysteriously lisa and david wasnt answering there phones and me and my other friends were waiting on them because we were all apost to hang out. Please someone help me out on what to do?