Yes. Because nothing to suggest a third party was involved means just that.
If there is no evidence, no body, no sign of a struggle, no blood, that kind of thing then there is nothing to suggest a third party involvement.
And that will remain the case until they find evidence to suggest otherwise (not just in this case but any case where a missing person is reported).
But if the police are asking for dash cam footage etc then they clearly haven’t ruled out the possibility.
And it’s also worth remembering that a third party involvement doesn’t necessarily mean someone who brought her to harm, it could be someone who was helping her disappear, could be another man for instance, or a friend, someone who knew what her plans were. Or it could be something more sinister.
It’s also entirely possible that the dog wasn’t left there but was put there.