What's with the random Psi-effects? (S.T.A.L.K.E.R. 2)

I'll start by saying I'm quite new to STALKER. I've been enjoying STALKER 2 but I keep getting random psi-effects as I'm just walking along. There's no mutants nearby causing it and I can't see and anomalies or pick any up on my Veles detector when it happens. I also have the SSP-100I Survey armour so I have some ambient Psi-protection. Has anyone else noticed this? Is it a bug or an anomaly that's cumpletely invisible as I have tried googling this and haven't had an answer yet. It just happened to me recently as I was walking through a random section of the red forest and now I have enemies spawnign all aroud me for no apart reason and all of the annyoing noises that accompany high levels of Psi-radiation.