This section on Wikipedia covers the history of the concerns. I wouldn't say there's a definitive smoking gun that's public knowledge. Instead, there are several instances of vulnerabilities discovered in Huawei systems that may have been taken advantage of (by the PRC or other parties). My guess is that there's more to a few of these stories that only the intelligence community knows about.

One other point of concern from Western governments is that Chinese law compels Huawei to cooperate with Chinese intelligence services, which presents a vulnerability whether that law has been taken advantage of or not. This is much different than say, Apple, which has refused (or it has been simply impossible due to E2E) for them to cooperate.

https://en.wikipedia.org/wiki/Criticism_of_Huawei#Espionage_...