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You may not want to burden the your daughters about this. This is an issue b.w you an him. It does sound that his behavior is somewhat odd (couples ought to share things b/w each other) You have a right to inquire about his behavior and he out of respect if nothing else should be honest and let you know about it.
It is also somewhat out of character for the woman to ignore you and not invite you or include you as that is quite disrespectful (but some people are like that) you can not assume that they are having something going on unless you find solid evidence.
As far as being angry, try to remain composed when you talk to him or confront him, otherwise he may try to use it as an excuse of not giving you info and may say that you're taking it too emotionally.