Hi I was just wondering if I store junk at the Power Armour docking station (yellow bar thingy) will the stuff just stay there or does it get "Stolen" ie disappears?

I've never had a problem with it. Been storing stuff there since day one.

If you're still concerned about losing your items, try putting something there you don't mind losing and leave the area for a few days (maybe a week), then come back and check on it.

I have not had anyone taking items from the workbenches in diamond city.

You could test it though. (save before trying) Put a high-end weapon in that workbench with the appropriate ammo. Make war with the population by taking a few pop-shots at them. Wait, dodge and see what happens.