I am so sorry you are going through this.
No you don't HAVE to snoop and if you can walk away and close the door on this knowing your relationship is over without ever needing answers then do just that.
However, most people want to know what it is they are dealing with - regardless if the outcome will make an ounce of difference to the next course of action.
I admire anyone able to leave at the stage you are at without knowing what it is thats behind the decline of the relationship. For some it does not matter why or the cause or what contributed to theend of a relationship. I on the other hand - having been in a similar situation have to say is not for me. I NEEDED to know what the hell had been going on and have to say 10 years on from my situation I am yet to meet a person that has ever said to me " I left/ended it because he was upto something behind my back and had already checked out of relationship but I dont know why".
If you are able - then go now and know you will never look back and wonder or ever ponder over why exactly but if you feel you need answers then please dont think you are lowering yourself to snoop to get those answers.
The fact is - whether you like it or not, you have been dragged into a shit situation (whatever exactly it is)through no fault of your own but because of the actions of your dishonest/secretive DH. The sad fact is - he is 99.9% never going to tell you what he is/has been upto off his own back or over some polite conversation over coffee.
The only time most of these people (men and women) in this situation ever spill the beans is when confronted with evidence they simply cannot refute and even then they will try (often referred to Damage limitation on alot of relationship threads). He may start with just looking at a dating site before eventually leading onto chatting online to someone, meeting them "just the once " etc etc.
If you feel the need to snoop then do. You have invested time, love and energy into this relationship and if you want answers you deserve them.
I am not encouraging you to snoop because it is a negative course of action and does not feel good and can become obsessive as from experience you usually find snippets and have to add it all together and then sometimes still its does not always the build the picture you imagine. It is a shitty thing to do - for yourself on an emotional level, but it is also a process that can (sometimes not always) help you see things, accept things and get some answers.
How the hell you do it when everything is password protected I dont know. In my situation I purely got lucky with a slip up on my partners side - he left his new phone unlocked and I just lucky that day - otherwise I would never have broken the codes/pins and passwords. Other things worth checking out is car - door panels the lot, suit pockets anywhere he may hide things. When I was desperate and googling I found anecdotes from woment who had the floor boards up/bath panels off revealing all kinds of stuff their partners/Husbands were upto. Another thing is to keep a diary of his timings and actions and where he says he is and when. There was a website I found (10 years ago now so dont recall it sorry)m where it suggested buying a seperate diary and noting down on each day his actions and tiings and checking them against train tickets/car mileage etc. Once you have established any pattern - from this I established my partner like Tuesdays for his philandering you can then decide on the next course of action - which may be to follow and confront or send someone else to watch for you.
None of this is ideal. I stopped short of confronting anyone and causing a scene. My snooping led me to a point where I had seen enough - I had the pretty much the whole picture and I just reached a point where I didnt want to snoop anymore. I am not sure I would have reached that point of acceptance without at least trying and uncovering some stuff. We are all different but please if you do decide to snoop - take time to stand back and be kind to yourself and remind yourself what exactly you need/want to know and dont get too sucked into it all. A good RL friend to confide in and keep you grounded is something I would strongly suggest.
In a perfect world you would walk away, never look back , never ponder and never wonder why, who, when , where - I admire any person able to do that but I could not, I needed to know.
Hugs - I am so sorry you are going through this shit.