Red flags are literally used to signal a problem and it can also be [used as a metaphor](https://en.wikipedia.org/wiki/Red_flag_(idiom)) to convey that the is a particular problem required attention or something that needs to be taken into account, or signals something problematic that needs attention.

For example (from a quick search on the Internet):

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> **Controlling behavior is a red flag in a relationship**. "[...] these people or simply feel the need to control where you go and who you associate with, limiting your world to allow in only what is important to them". From *[10 Relationship Red Flags](https://www.psychologytoday.com/us/blog/in-flux/201407/10-relationship-red-flags)*
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> **Rudeness in a job interview is a red flag**. "Rudeness is a pretty simple red flag to spot. It can come in a couple of different guises though...". From *[The 8 Biggest Interview Red Flags](https://beamery.com/blog/interview-red-flags)*
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> If you are trying to hire a contractor and they ask for too much money upfront, that is a red flag. "[...] a contractor who asks for more than 15% upfront should elicit an uh-oh feeling." From *[Beware of These 8 Red Flags When Hiring a Contractor](https://www.realtor.com/advice/home-improvement/hiring-contractor-red-flags/)*
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I doubt that the literal translation, "bandera roja" is used in Spanish (never heard of it), but it could be the case.

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> Si un contratista te pide demasiado dinero por adelantado, eso es una *bandera roja*.
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> Si tu pareja te controla los mensajes del teléfono y el email, eso es una *bandera roja* en la relación.
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How do you translate the idiom "red flag" into Spanish?