This comes three weeks after Edwards and Malik's break-up became public.
Thirwall, 22, was promoting the band's new single "Black Magic" in Arizona, when she photographed with the message, reported Female First.
Other fans have insisted the furore has been exaggerated as the reverse of the sign read "Stay Strong Perrie," as a message of support following the shocking break-up, which was reportedly done by text message.
Edwards, 22, is reportedly set to open up about the break-up for the first time in a tell-all interview with British comedian James Corden.
