Where, besides the nginx config and php code, can the redirect be buried?

Question

Nginx

the site redirects to the http version if the referral specifies the https version

GET /articles HTTP/1.1
Host: rt.ru
Connection: keep-alive
Upgrade-Insecure-Requests: 1
User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/64.0.3282.189 Safari/537.36 Vivaldi/1.95.1077.60
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8
Referer: https://rt.ru/articles/
Accept-Encoding: gzip, deflate, br
Accept-Language: ru-RU,ru;q=0.9,en-US;q=0.8,en;q=0.7
Cookie: rdnt=1; b=b; PHPSESSID=bplmforfu756apkt65ai2vpa91; _ym_uid=15232512571067320353; rdnt=1; b=b; caltat=99f95242a5944010a9e2a172651396cd; phpbb3_nb9q6_u=6003; _ym_isad=2; phpbb3_nb9q6_k=729e2e8109b4d342; phpbb3_nb9q6_sid=2fb03bafbc5e522a0ff69fdea6a2be11; __lx205948_load_cnt=13; __lx205948_load_tmr=1524246959733; __lx205948_load_tmr_pre=1524285228203; last_visit=1524276304609::1524287104609


HTTP/1.1 301 Moved Permanently
Server: nginx/1.6.0
Date: Sat, 21 Apr 2018 04:00:34 GMT
Content-Type: text/html; charset=iso-8859-1
Content-Length: 236
Connection: keep-alive
Location: http://rt.ru/articles/

GET /articles/ HTTP/1.1
Host: rt.ru
Connection: keep-alive
Upgrade-Insecure-Requests: 1
User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/64.0.3282.189 Safari/537.36 Vivaldi/1.95.1077.60
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8
Accept-Encoding: gzip, deflate, br
Accept-Language: ru-RU,ru;q=0.9,en-US;q=0.8,en;q=0.7
Cookie: rdnt=1; b=b; PHPSESSID=bplmforfu756apkt65ai2vpa91; _ym_uid=15232512571067320353; rdnt=1; b=b; caltat=99f95242a5944010a9e2a172651396cd; phpbb3_nb9q6_u=6003; _ym_isad=2; phpbb3_nb9q6_k=729e2e8109b4d342; phpbb3_nb9q6_sid=2fb03bafbc5e522a0ff69fdea6a2be11; __lx205948_load_cnt=13; __lx205948_load_tmr=1524246959733; __lx205948_load_tmr_pre=1524285228203; last_visit=1524276322132::1524287122132

HTTP/1.1 200 OK
Server: nginx/1.6.0
Date: Sat, 21 Apr 2018 04:02:21 GMT
Content-Type: text/html; charset=cp1251
Transfer-Encoding: chunked
Connection: keep-alive
X-Powered-By: PHP/5.4.30
Expires: Thu, 19 Nov 1981 08:52:00 GMT
Cache-Control: no-store, no-cache, must-revalidate, post-check=0, pre-check=0
Pragma: no-cache

there is no redirect either in the php code or in the nginx config