High Sec space only has DED 1/10 to 4/10 sites but escalation from Anomaly combat site can spawn DED 5/10 site in High Sec space. I believe Low Sec space has DED 1/10 up to 7/10 and those higher level sites definitely don’t spawn in High Sec space.
Anyway, nobody really knows for sure since CCP hasn’t released any info on their respawn mechanics. I think the sites are restricted and will only respawn within their immediate Constellation, dependent on the systems security level. For example a DED 6/10 site in a Low Sec system will not respawn in a High Sec system located next to it.
Also the sites seem to have a tendency to ‘gather up’ in dead-end systems that don’t get visited much by players. I’ve seen up to a couple dozen Signatures with twice as many Anomaly sites in those dead-end systems.
Now I don’t know whether it’s due to those sites respawning there after being completed elsewhere or if they just randomly spawned there, it’s probably a combination of the two which would help generate a large amount of sites being gathered there. Also exploration sites have a ‘shelf life’ of 3 to 5 days of staying present in a system before they respawn elsewhere. That would also explain why dead-end systems usually have a large amount of exploration sites.
One thing is for sure, if you can’t find any sites in regular travel routes, there’s a very high chance you’ll find them in backwater ‘off the beaten path’ dead-end systems.