Only when the rubber pipe from Block to tell-tale outlet was pulled off accidentally whilst I was mucking about under the cowling
Won't do that again. Luckily I only ran it up on the muffs before I noticed, although I did panic at the lack of tell-tale until I figured it out.
I presume the rubber seal is fitted OK on your engine.
Worth also checking somebody hasn't removed a water temp/pressure gauge or similar without blocking off the hole before you brought the engine.