Implantation could belater than you think. 

If a woman implants at 8dpo, with a 'non pregnant' hCG level of 1 [Which is totally normal] it would rise as follows

8dpo - 1miu hcg 10dpo - 2miu 12dpo - 4miu 14dpo [Day AF is typically due] - 8miu 16dpo - 16miu 

So this lady wouldn't finnd out until 15dpo on a very senstive 10miu pregnancy test, superdrug own or possibly asda own. Obviously if she implants LATER than 8dpo [which is normal] she will find out even later.

Now, Take a lady who implants at 8dpo, But has a'Non pregnant' hCG level of 5. Hers looks like this;

8dpo - 5miu 10dpo- 10miu - Could find out here on superdrug own, internet cheapie 12dpo - 20miu - Could find out here with freedom cheapie strip/most poundland tests and asda own] 14dpo - 40miu [day period is typically due] Most tests would pick it up here"