laravel asset() method doesn't return https

isn't it secure_asset you're looking for?

https://laravel.com/docs/5.3/helpers#method-secure-asset

As you can see from GitHub the asset method is calling getScheme to determine what the scheme should be.

https://github.com/illuminate/routing/blob/master/UrlGenerator.php#L303

public function formatScheme($secure = null)
{
    if (! is_null($secure)) {
        return $secure ? 'https://' : 'http://';
    }
    if (is_null($this->cachedScheme)) {
        $this->cachedScheme = $this->forceScheme ?: $this->request->getScheme().'://';
    }
    return $this->cachedScheme;
}

So if you don't provide the asset 2nd parameter $secure then it uses the request scheme. Otherwise you can provide $secure to force the desired scheme regardless of what is the scheme in the request.

If you look at the code you'll see that if $secure is null and no cache is set than the cache is set to the request scheme (i.e. $this->request->getScheme()) and therefore returned.