Person ID = 1

Access-Control-Allow-Origin: *
Access-Control-Expose-Headers: ETag, X-RateLimit-Limit, X-RateLimit-Remaining, X-RateLimit-Reset, Retry-After
Connection: keep-alive
Content-Encoding: gzip
Content-Length: 1942
Content-Type: application/json;charset=utf-8
Date: Fri, 08 Jun 2018 17:48:06 GMT
Server: openresty
Vary: Accept-Encoding
X-RateLimit-Limit: 40
X-RateLimit-Remaining: 18
X-RateLimit-Reset: 1528480093

Person ID = 9

Access-Control-Allow-Origin: *
Access-Control-Expose-Headers: ETag, X-RateLimit-Limit, X-RateLimit-Remaining, X-RateLimit-Reset, Retry-After
Connection: keep-alive
Content-Length: 646
Content-Type: application/json;charset=utf-8
Date: Fri, 08 Jun 2018 17:49:26 GMT
ETag: 0ff39bbbf981ac0151d340c9aa40e63e
Server: openresty
X-RateLimit-Limit: 40
X-RateLimit-Remaining: 27
X-RateLimit-Reset: 1528480174

So far out of 12,000 calls, 4000 are missing ETags.

Thus far, I can't work out what distinguishes one from another in terms of whether or not they get an ETag.

Looking at it, I suspect it may be down to size. e.g. if it is over a certain size, it gzips it. If it gzips it, it doesn't have an ETag.