No, what this creates is 10'-pole players. Which drags down the game and makes people paranoid, rules-lawyery and all around un-fun. As a DM, I never want to look like a jerk. I give my players fair chance and if they screw it up that's on them, they're not mind-readers and I'm not psychic.
You're missing the trees for the forest. Every threat does telegraph itsself, but not always in obvious ways. That's what needs to be established and what I feel like was completely missing from your game. There are plenty of ways to make stealth really useful while also making the players aware that they may be walking into danger.
Ankhegs are giant(large size) bugs that burst forth from the ground and consume unlucky passersby. Their territory is going to be fairly simple, open terrain. Large, rooted plants and massive boulders will keep them from being able to get ideal positioning. There will also be signs of their presence if they are common in the area, man-size burrows can be found around them from both where they burst out of the ground and where they come out of it naturally for whatever reason. The ground is also likely to be quite loose, possibly even having sunken "trails" from where their tunnels have collapsed over time.
There are lots of comparative creatures that are "ambush predators" in real life, but signs of their presence are fairly obvious and those creatures are usually quite small (insects). A 8ft+ insect is going to have a lot of trouble being subtle about its presence in the area.
This is what I'm getting at. The presence of something potentially bad in the area is "telegraphed" by burrows and disturbed earth in open, plain areas.