Is it haram to have sex outdoor without anyone looking? For example a car with closed windows or in a hotel. (Only between you and your spouse.)
This seems to hinge primarily on (a) the chance of getting caught, and (b) whether or not it's legal.
A husband should not have intercourse with his wife when it is possible for someone else to see them or to hear their noise when copulating. Also, he should not kiss her or touch her in front of the people. Imaam Ahmad may Allaah have mercy upon him said: "For me, it is desirable that he should conceal all of this (i.e. do this in privacy only). -- Ibn Qudaamah, sourced from IslamWeb.net
There's the possibility of being "caught" as the act proceeds (perhaps in person, or perhaps on security footage; maybe recorded on a mobile phone). While perhaps exciting, it's worth keeping in mind, if you are caught:
whoever inadvertently witnesses it does not consent to participate in your sex act in any way (including viewing);
it may be a minor (i.e., a child) who inadvertently witnesses your sex act;
you and your partner may be exposing your awrah;
Muslims should have some kind of shyness (hayaa):
Abu Sa'id Khudri reported that Allah's Messenger was more modest than the virgin behind the curtain (or in the apartment), and when he disliked anything, we recognised that from his face. (sunnah.com)
Having sex in public is a crime in certain circumstances, and Muslims are expected to obey the law:
Muslims must adhere to the laws of any country they live in, whether in the west or the east, as long as the law is not in contradiction with one’s religion. -- Mufti Muhammad ibn Adam
And it seems reasonable to believe that Muslims are expected to obey the law even if they are 100% guaranteed to get away with disobeying the law.
In a UK context, for example, BBC write:
Offences that could be committed in a 'public sex environment'
Outraging public decency contrary to common law
Behaviour that is likely to cause harassment, alarm or distress to other users contrary to the Public Order Act 1986
Offences of exposure - if the person exposes themselves to someone intending that someone will see them and be caused alarm or distress
Sex in a public toilet (Section 71, Sexual Offences Act, 2003)
The above applies to England and Wales although there are some similar provisions in Scotland and Northern Ireland
It's a complicated issue. Outdoor sex in the middle of a forest is clearly at the other end of the spectrum from sex on a crowded beach. But both can be interpreted as sex in public places.
A person in a public place must not wilfully expose his or her genitals, unless the person has a reasonable excuse. -- Queensland, SUMMARY OFFENCES ACT 2005 - SECT 9
A person must not wilfully and obscenely expose the genital area of his or her body in, or within the view of, a public place. -- Victoria, SUMMARY OFFENCES ACT 1966 - SECT 19
Section 29. Indecent acts in public place.
Any person who, contrary to Islamic Law, acts or behaves in an indecent manner in any public place shall be guilty of an offence and shall on conviction be liable to a fine not exceeding one thousand ringgit or to imprisonment for a term not exceeding six months or to both.